The hexosamine biosynthetic pathway (HSP) is a minor branch of the glycolytic pathway, diverting 3-5% of cellular glucose toward the synthesis of UDP-GlcNAc, which is either transported to the golgi and used in the synthesis of complex glycans or remains in the cytoplasm where it is the obligatory substrate for O-GlcNAc Transferase (OGT). OGT is the sole known enzyme to catalyze the transient glycosylation of serine and threonine residues on many nuclear and cytoplasmic proteins (termed O-GlcNAcylation). This post-translational modification is dynamic and is a general method, like protein phosphorylation, of signal transduction. There are also other glycosylating enzymes that post-translationally modify molecules in the body.
Excess flux through the HSP has been implicated in both early (insulin resistance) and late (nephropathy, microvascular damage) stages in the course of diabetes mellitus, both in vivo and in vitro. Diabetes involves a deficiency in the availability and/or utilization of insulin. Insulin is a hormone produced by the pancreas and is necessary for cells to utilize glucose. Insulin resistance is a condition in which muscle, fat, and liver cells do not use insulin properly. As a result, the pancreas produces more insulin, which also cannot be properly used. Eventually, the pancreas cannot keep up with the body's need for insulin, and excess glucose builds up in the bloodstream. Thus, in insulin resistance, there may be high levels of blood glucose and high levels of insulin circulating in the bloodstream at the same time.
Experiments have shown that insulin resistance due to increased hexosamine flux is caused by hyper O-GlcNAcylation. Diabetics have increased production of two adipokines directly responsible for vascular injury, plasminogen activator inhibitor-1 (PAI-1) and transforming growth factor β1 (TGF-β1). Transcription of both of these proteins is decreased in cell culture when levels of O-GlcNAcylation were decreased. The molecular mechanism for this is known; increased transcription is mediated by the O-GlcNAcylation state of the transcription factor Sp1. OGT activity and levels of O-GlcNAcylation and the activity of additional glycosylating enzymes have also been implicated in other disease states, such as Alzheimer's disease and cancer.
The ppGalNAcT family of glycosyltransferases initiates core-type O-glycan formation, that may be elaborated further by glycan linkages, contributing to selectin ligand-dependent control of leukocyte trafficking, regulation of CD8+ T-cell aoptosis, and sensitivity to colitis and preventing the onset of Tn syndrome. The prototype ppGalNACT family member is ppGalNAcT-1, which is expressed at high levels in most tissues and cell types. Loss of ppGalNAcT-1 activity in a mouse model resulted in bleeding disorder and impaired IgG production. ppGalNAcT-1 has been shown to support normal homeostatic physiology and inflammatory response [(Term et al., Mol Cell Biol 27: 8783-8796 (2007)].